Saikyou no Maou ni Kitaerareta Yuusha Isekai

Without possessing the unique skill blessed by the goddess, a boy named Kujou Kyouya failed to save the world. He was ridiculed as a “failed hero” at the interdimensional-level academy where returned heroes who had saved many worlds gathered. However, he became the strongest and most unique hero, having trained for 30,000 years in another world with the most evil demon king, Felice, who has destroyed 3,000 worlds! Now Kyouya’s unique skill is so powerful that he cannot even be defeated by the strongest heroes! The legend of a boy trained by the most evil demon king, begins!

A Brave Man Trained by the Worst Demon King Unrivaled in the School of Returnees From Another World; Isekai Kikansha-tachi no Gakuen de Musou Suru; Sai Kyō no Maō ni Kitae Rareta Yūsha, Isekai Kikan-sha-tachi no Gakuen de Musō Suru; Saikyou no Maou ni Kitaerareta Yuusha; Saikyou no Maou ni Kitaerareta Yuusha Isekai Kikanshatati no Gakuen de Musou Suru; 最凶の魔王に鍛えられた勇者、異世界帰還者たちの学園で無双する